Upgrade to 5.43-RC causes Fatal `Class 'CRM_Contact_Form_Search_Custom_Base' not found`

Upgrade to today's RC from 5.39.3 results in fatal errors and being unable to load the Admin UI or run cv commands.

cv flush results in

 [Symfony\Component\Debug\Exception\FatalThrowableError]
  Class 'CRM_Contact_Form_Search_Custom_Base' not found

Removing all files (except .htaccess) under templates_c does not correct this error.

any other cv command results in the same error. wp-admin/admin.php?page=CiviCRM&q=civicrm%2Fadmin%2Fsetting%2FupdateConfigBackend&reset=1cannot load trying to clear from /wp-admin/admin.php?page=civi_options also fails

log shows:

[22-Oct-2021 12:49:27 UTC] PHP Fatal error:  Uncaught Error: Class 'CRM_Contact_Form_Search_Custom_Base' not found in /home/wpcv/public_html/wp-content/uploads/civicrm/ext/dataprocessor/CRM/DataprocessorSearch/Form/Search/Custom/DataprocessorSmartGroupIntegration.php:29
Stack trace:
#0 /home/wpcv/public_html/wp-content/plugins/civicrm/civicrm/CRM/Core/ClassLoader.php(220): require_once()
#1 [internal function]: CRM_Core_ClassLoader->loadClass('CRM_Dataprocess...')
#2 /home/wpcv/public_html/wp-content/plugins/civicrm/civicrm/vendor/symfony/event-dispatcher/EventDispatcher.php(214): spl_autoload_call('CRM_Dataprocess...')
#3 /home/wpcv/public_html/wp-content/plugins/civicrm/civicrm/vendor/symfony/event-dispatcher/EventDispatcher.php(44): Symfony\Component\EventDispatcher\EventDispatcher->doDispatch(Array, 'civi.dao.postUp...', Object(Civi\Core\DAO\Event\PostUpdate))
#4 /home/wpcv/public_html/wp-content/plugins/civicrm/civicrm/Civi/Core/CiviEventDispatcher.php(198): Symfony\Component\EventDispatcher\EventDispatcher->dispatch('civi.dao.postUp...', Object(Civi\Co in /home/wpcv/public_html/wp-content/uploads/civicrm/ext/dataprocessor/CRM/DataprocessorSearch/Form/Search/Custom/DataprocessorSmartGroupIntegration.php on line 29

I had updated this site last week to 5.43-RC but I rolled back to production (5.39.3) for other reasons and today's RC build fails.

I can update to 5.42.1, so the issue appears to be in the RC.

I have not tried disabling DataProcessor before upgrade. I'll try thta and report back.